Update V8 to version 4.6.21.

Summary of changes available at:
https://chromium.googlesource.com/v8/v8/+log/49d3837a..b2dfa5cd

Please follow these instructions for assigning/CC'ing issues:
https://code.google.com/p/v8-wiki/wiki/TriagingIssues

Please close rolling in case of a roll revert:
https://v8-roll.appspot.com/

TBR=hablich@chromium.org,machenbach@chromium.org,yangguo@chromium.org,vogelheim@chromium.org

Review URL: https://codereview.chromium.org/1250853003

Cr-Commit-Position: refs/heads/master@{#339772}

DEPS

@@ -38,7 +38,7 @@ vars = {
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling V8
   # and whatever else without interference from each other.
-  'v8_revision': '49d3837a3f7beb29b8afea6f8be96216386e6ed6',
+  'v8_revision': 'b2dfa5cdffacb2725978ed4789cb5db85783710e',
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling swarming_client
   # and whatever else without interference from each other.